1. Field of the Invention
The present invention relates to a process for production of biphenyl tetracarboxylic acid-based polyimide molded bodies with satisfactory molded body properties and productivity, and to the polyimide molded bodies produced thereby.
The invention is characterized by excellent productivity by allowing molding of a plurality of bodies at one time, by giving polyimide powder molded bodies with density that is close to the true density of the polyimide resin powder starting material, and by giving molded bodies with high toughness and mechanical strength and substantially no anisotropy of the linear expansion coefficient.
2. Description of the Related Art
Pyromellitic acid-based polyimide powder molded bodies obtained from a pyromellitic acid component and 4,4xe2x80x2-diaminodiphenylene ether have been widely used in the prior art as polyimide powder molded bodies because of their high toughness and satisfactory cutting workability.
However, pyromellitic acid-based polyimide molded bodies have high moisture absorption, considerable out gas and low chemical resistance and dimensional stability.
3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid-based polyimide powder molded bodies have therefore been proposed.
Examples of such 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid-based polyimide powder molded bodies are described, for example, in Japanese Unexamined Patent Publication No. 57-200452 (Japanese Examined Patent Publication No. 2-48571) and Japanese Unexamined Patent Publication No. 57-200453, wherein there are obtained heated/compressed molded bodies of aromatic polyimide powder with an imidation rate of 95% or greater obtained by polymerization and imidation of a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid component and an aromatic diamine component in N-methyl-2-pyrrolidone.
Also, polyimide powder molded bodies containing inorganic powder such as fine particulate graphite are described in Japanese Unexamined Patent Publication No. 63-81160.
According to these publications, these polyimide powder molded bodies exhibit excellent mechanical strength.
However, high-strength polyimide powder molded bodies with high heat resistance have been shown to have certain drawbacks, possibly due to their low elongation, such as breakage during molding and poor suitability for molding into complex shapes, when the molded bodies are subjected to secondary working into various shapes by cutting or the like; in other words, their toughness and cutting workability are low.
For this reason, it has been attempted to improve the powder fusing properties during hot compression molding in order to increase the elongation and mechanical strength of the molded bodies.
For example, a method of compression molding of polyimide powder obtained by mixing a thermoplastic polyimide with a polyimide obtained from a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid component and an aromatic diamine component has been attempted, but this has been associated with problems such as difficulty in obtaining a uniform mixture of the two components with completely different properties, the fact that the mechanical strength and elongation of the resulting molded bodies have still not reached a satisfactory level, and the fact that the heat resistance is instead reduced.
It has also been attempted to first extract polyamic acid powder (aggregates) and subject it to heating, drying and pulverization to obtain polyimide powder, and then subject this to compression molding to obtain a molded body. However, it has been found difficult to control the heating temperature in order to prevent production of powder aggregates when the polyamic acid powder is heated for imidation, while metal impurities also tend to be included in the polyamic acid powder, so that the process is not practical.
There have hence been proposed processes for high-temperature, high-pressure hot compression molding of polyimide resin powders obtained by polymerization and imidation of p-phenylenediamine with 3,3,4,4xe2x80x2-biphenyltetracarboxylic acid, its ester or its dianhydride and 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its ester or its dianhydride, for example, a process for production of polyimide molded bodies involving simultaneous heated firing and pressure/compression with a uniaxial press at a molding temperature of 450xc2x0 C. and a molding pressure of 3,000 kgf/cm2.
The polyimide molded bodies obtained by these molded body production processes exhibit high strength, but the molded bodies show linear expansion coefficient anisotropy and also have low productivity, such that problems arise in terms of cost for mass production.
It is therefore an object of the present invention to provide a process for production of polyimide molded bodies that exhibit improvement in the properties of high moisture absorption, considerable out gas and low chemical resistance and dimensional stability exhibited by pyromellitic acid-based polyimide powder molded bodies comprising a pyromellitic acid component and 4,4xe2x80x2-diaminodiphenyl ether, while also exhibiting high mechanical strength and high productivity, as well as the polyimide molded bodies obtained by the process.
In other words, the invention provides a process for production of polyimide molded bodies which comprises a step in which a polyimide resin powder obtained by polymerization and imidation of p-phenylenediamine with 3,3,4,4xe2x80x2-biphenyltetracarboxylic acid, its ester or its dianhydride and 2,3,3xe2x80x2,4xe2x80x2-biphenyltetracarboxylic acid, its ester or its dianhydride is molded at a pressure of approximately 800-5,000 kgf/cm2, a step in which the pressurized body is subjected to calcination at about 450-550xc2x0 C. under low pressure and preferably in the absence of pressure, a step in which the calcined molded body is vacuum encapsulated into a metal capsule, and a step in which the encapsulated molded body is isotropically subjected to hot compression at approximately 460-550xc2x0 C. in an inert atmosphere and preferably in an argon atmosphere.
The invention further provides a polyimide molded body produced by the aforementioned process wherein the density of the molded body is in the range of 1.44-1.48 g/cm3.
The invention still further provides a polyimide molded body which is obtained by molding polyimide powder containing at least 70 mole percent of a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride component and at least 70 mole percent of a phenylenediamine component, by a molding method involving the HIP method, and which has a flexural strength of approximately 85 MPa or greater.
The invention still further provides a process for production of polyimide molded bodies which comprises molding polyimide powder containing at least 70 mole percent of a 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride component and at least 70 mole percent of a phenylenediamine component, by a molding method involving the HIP method, wherein the resulting molded bodies have a flexural strength of approximately 85 MPa or greater.